Field of the Invention
The present invention relates to a roller unit for applying release oil to the heating roller of a fixing device for use in monochrome and color printers, copiers, facsimiles, etc., and also relates to an image formation apparatus incorporating the roller unit.
Color toner for use in image formation apparatuses, such as a copier, printer and the like, for forming full-color images by an electrophotographic image formation process is required to have superior transmittancy at the time when the toner is fixed to a recording medium. Color toner including a binder resin, being fused quickly and having a low viscosity at the time of fusion is used to attain this superior transmittancy. A method of fixing this color toner by using a fixing device having a heating roller of silicon rubber has been employed conventionally. In the above-mentioned fixing device, the surface of the heating roller is formed of silicon rubber. Furthermore, the fixing device uses color toner including a binder resin being fused quickly and having a low viscosity at the time of fusion. For these reasons, hot offset, a phenomenon wherein fused color toner is attached to the surface of the heating roller, is apt to occur. To prevent hot offset, a release agent, such as silicone oil, having a high affinity for silicon rubber and being relatively inexpensive, is applied abundantly to the surface of the heating roller.
A release ingredient included in the toner itself has recently been used to prevent hot offset. In this case, it is still necessary to apply a release agent, although the application amount of the release agent is small.
Various roller-shaped members have been proposed as a member for applying oil to the fixing device or the like. As an example of this type, Japanese Patent Publication No. Hei 1-60144 discloses a roller-shaped member comprising a pipe with a plurality of pores disposed in the external circumferential portion thereof, release oil provided at the central portion of the pipe, and heat-resistant felt made of Nomex or the like wound around the external circumference of the pipe. Japanese Laid-open Utility Model Application No. Sho 61-104469 discloses a roller-shaped member covered with a porous material, such as paper, on which heat-resistant felt is wound to control leak amount of oil and diffuse oil in the axial direction of the roller, and uniformly apply oil to its entire surface. Furthermore, Japanese Laid-Open Patent Application No. Hei 5-123623 discloses a member having a silicon sponge layer or the like used as an intermediate layer to provide elasticity. Moreover, Japanese Laid-open Patent Application No. Sho 61-104469 discloses a roller-shaped member comprising a heat-resistant porous material instead of the heat-resistant felt.
In these conventional arts oil exudes from the felt or porous material on the surface, and is directly applied and transferred to the fixing roll. Therefore, it is difficult to control the application amount of the oil. To make the application amount of oil controllable, Japanese Laid-open Patent Application No. Sho 61-183679 discloses a member wherein a heat-resistant microporous film layer is provided on the surface of a heat-resistant felt layer or a porous material. Japanese Laid-open Patent Application No. Sho 62-178992 discloses a member wherein a composite film filled with a mixture of silicone rubber and oil is used as an oil permeation control layer.
In a color electrophotographic apparatus, its photosensitive member is charged by corona discharge using a charger. Light signals are then cast to the photosensitive member to form electrostatic latent images for all colors. Next, toner for a first color, such as yellow toner, is used for development to obtain a visible image. A transfer material charged in the polarity opposite to that of the yellow toner is made contact with the photosensitive member to transfer the yellow toner image formed on the photosensitive member. Toner remaining after the transfer is removed by cleaning from the photosensitive member, and the photosensitive member is electrically erased. This completes the development and transfer of the first color toner.
The same operation as that for the yellow toner is carried out repeatedly for magenta toner and cyan toner. The toner images of these colors are superimposed on the transfer material to form a color image. The toner image obtained by the superimposing operation is transferred to paper charged in the polarity opposite to that of the toner, and fixed, thereby ending copying operation.
The transfer drum method is generally used as a color image formation method. In the transfer drum method, color toner images are formed in sequence on a single photosensitive member. A transfer material entrained about a transfer drum is rotated so as to be opposed repeatedly, to the photosensitive member, whereby the color toner images formed in sequence are superimposed and transferred. Furthermore, the continuous superimposing method is also used in general, wherein a plurality of image formation units are arranged in a line, and a transfer material is fed via belts and is passed through the image formation units so that color toner images are transferred in sequence.
A color image formation apparatus using the above-mentioned transfer drum method is disclosed in Japanese Laid-open Patent Application No. Hei 1-252982.
On the other hand, a conventional example of a color image formation apparatus using the continuous transfer method is disclosed in Japanese Laid-open Patent Application No. Hei 1-250970. This conventional example is provided with four image formation stations each comprising a photosensitive member, optical scanning means and the like to carry out four-color image formation. Paper fed via belts is passed through the lower portion of each photosensitive member, whereby color toner images are superimposed.
Another method of forming a color image by superimposing different color toner images on a transfer material is disclosed in Japanese Laid-open Patent Application No. Hei 2-212867. In this method, color toner images formed in sequence on the photosensitive member are superimposed once on an intermediate transfer material, and the toner images on the intermediate transfer material are transferred to paper all together.
However, in order to attain stable control of the permeation amount of oil at the oil permeation control layer in these conventional examples, the amount of oil supplied to the oil permeation control layer must be controlled constant. Furthermore, the oil holding layer thereof is desired to have a two-layer structure comprising an oil supply layer and an oil diffusion layer to accomplish stable oil application for a long term. In this case, a difference in osmotic pressure is required between the two layers so that oil can move. In addition, the porosity (the ratio of the volume of pores to the total volume) of the oil supply layer is required to be higher than the porosity of the oil diffusion layer. The porosity of the oil supply layer is also required to be high so that the oil application roller can have a long service life. However, if the porosity of the oil supply layer is high, its oil holding capability lowers, thereby being apt to cause oil leakage. Moreover, if an oil tank is used as the oil supply layer, the risk of oil leakage may become higher.
Furthermore, in the event that release oil makes contact with the microporous film or the composite film of the oil permeation control layer, the pores in the microporous film are closed by oil. If a large amount of gas is generated abruptly, it is difficult for the gas to pass through the pores. If the gas is forcibly passed through with a pressure, then a considerable pressure for example, of about 50 kgf/cm.sup.2 is required. For this reason, higher pressure is required if the diameter of the pores in the microporous film is made smaller to decrease the permeation amount of oil. In addition, in the case of the above-mentioned composite film, it is difficult to allow gas to pass through without damaging the film.
Furthermore, a steam pressure rises because of moisture absorbed in oil and the porous material depending on the change in the temperature of the fixing device. Moreover, a pressure also rises in the oil supply layer when the oil itself expands owing to heat. Because of these pressure rise, the oil permeation control layer undergoes an excessive pressure. As a result, oil is liable to burst out, thereby increasing the first application amount of oil. In some cases, trouble may be caused, for example, the oil permeation control layer may be damaged or may swell like a balloon.
To prevent this trouble, vent holes for allowing the pressure inside the oil application roller to release are provided at the end portions of the oil application roller. In particular, the vent holes should be provided in the axial direction of the roller. In this case, however, oil inevitably leaks from the vent holes when the oil application roller is tilted during transportation or storage.
If the oil application roller is sealed completely by vacuum packing or the like to prevent oil leakage during transformation, oil leakage will not be caused. However, it is difficult to seal the oil application roller together with its housing unit. Although it is possible to use a method wherein only the oil application roller is packed so as to be sealed completely and then unpacked before use, ease of handling by the user is impaired in this case.
Furthermore, in the case of sealing the roller completely by winding a film or the like around the roller, if an adhesive is applied to the circumference of the film and bonded to the roller to attain tight sealing, a large force is required to remove the film at the time of unpacking. If the film is not removed carefully and properly, the film may be torn. In addition, if the adhesive remains on the oil application roller, nonuniform oil application may be caused.